1. Field of the Invention
The present invention relates to an ion implantation apparatus and an ion implantation method, and particularly to the ion implantation amount control of an ion implantation apparatus having a function of compensating for a change in the dose amount caused by charge conversion during ion implantation using measurement of the degree of vacuum.
2. Description of Related Art
In a semiconductor-manufacturing process, it is normal to carry out a process in which ions are implanted in a semiconductor wafer for the purpose of changing conductivity, changing the crystal structure of the wafer, and the like. An apparatus used in the process is referred to as an ion implantation apparatus, and has a function of forming an ion beam which is ionized by an ion source, and then accelerated and a function of irradiating the ion beam to the entire surface of the semiconductor wafer using beam scanning, wafer scanning, or a combination thereof.
In an ion implantation process in the semiconductor-manufacturing process, generally, it is necessary to uniformly implant a target dose amount in a wafer surface, and it is normal to control the ion implantation apparatus for that purpose.
In such an ion implantation apparatus, a control operation is carried out by measuring an ion beam current value and computing an implantation dose amount from the valence of an ion implanted into a wafer, but a vacuum correction function through which the ion beam current value is corrected is proposed in order to carry out an accurate control (see JP-A-2000-11942 (Patent Document 1)). The reasons why the vacuum correction function is used are as follows.
In the ion implantation apparatus, in order to improve the beam transportation efficiency or prevent the charging up of a wafer into which ions are implanted, gas is intentionally introduced into a beam line from outside, and electrons are supplied through ionization or plasmatization of the introduced gas. A noble gas is mainly and frequently used as the introduced gas. A resist film is coated on a wafer into which ions are implanted, and an ion beam is irradiated to the resist film, thereby generating a gas caused by the resist film. Some of the introduced gas intentionally introduced into the beam line and a resist-induced gas generated through implantation of an ion beam into the wafer having the resist film stay in the beam line as residual gas. Particles in the ion beam collide with the residual gas, and convert charges in a certain proportion, thereby being neutralized. Since the particles in the ion beam which are neutralized through charge conversion cannot be measured as an ion beam current, the number of particles implanted into the wafer cannot be reliably measured such that control of the implanted dose amount becomes inaccurate. Therefore, the vacuum correction function is used in order to correct the effect of neutralizing the particles in the ion beam through collision of the ion beam with the residual gas and control the dose amount accurately.
In the vacuum correction function disclosed in Patent Document 1, a measured value of the ion beam current is represented by Im, the partial pressure value of the introduced gas introduced from outside is represented by PA, a vacuum gauge-measured value is represented by P, the partial pressure value of the resist-induced gas is represented by P-PA, the vacuum correction coefficient which indicates how easily the introduced gas of the ion beam is neutralized is represented by KA, and the vacuum correction coefficient with respect to the resist-induced gas is represented by K. In the vacuum correction function disclosed in Patent Document 1, additionally, an implanted ion beam current I0 which is supposed to be measured when the ion beam particles are not neutralized due to charge conversion is calculated, and the dose amount is controlled based on the above value. The above elements are assumed to satisfy the following formula.Im=I0×f(P)f(P)=exp[-KAPA−K(P−PA)]
Herein, the values of the vacuum correction coefficients KA and K included in the function f(P) of pressure vary depending on the kind of the ion, the accelerated voltage of the ion beam, and the kind of the introduced gas.
In the introduced gas introduced from outside and the resist-induced gas, the proportion of particles that are neutralized when the ion beam collides varies. Therefore, it is necessary to accurately estimate the partial pressure values of the introduced gas and the resist-induced gas respectively. Nevertheless, the vacuum correction function disclosed in Patent Document 1 has a problem regarding the estimation method.
In the vacuum correction function disclosed in Patent Document 1, a vacuum gauge-measured value in a state in which a resist-induced gas before ion implantation is not generated is used as the partial pressure value PA of the introduced gas, and the partial pressure value PA is assumed to be constant at all times even during ion implantation. In addition, a value P−PA which indicates the difference between the vacuum gauge-measured value P which continuously changes every second during ion implantation and the partial pressure value PA of the introduced gas is used as the partial pressure value of the resist-induced gas.
However, as described in detail below, in the ion implantation apparatus, ordinarily, structures operating during ion implantation into a wafer are present. In addition, the mechanical operations of the structures change the vacuum conductance of the beam line. This means that the partial pressure value PA of the introduced gas significantly changes with respect to time during ion implantation.
In addition, as described in detail below, the introduced gas intentionally introduced into the beam line is not necessarily one kind; however, in the vacuum correction function disclosed in Patent Document 1, consideration of the above point is lacking.
Due to the above, in the vacuum correction function disclosed in Patent Document 1, errors occur in the partial pressure value PA of the introduced gas and the partial pressure value P−PA of the resist-induced gas obtained from the difference from the vacuum gauge-measured value P respectively. As a result, the implanted ion beam current used to control the dose amount deviates, and it becomes impossible to accurately control the dose amount.
Therefore, in order to accurately estimate the partial pressure values of plural kinds of introduced gases and the resist-induced gas respectively, it is necessary to include a conductance effect and the kinds of the introduced gases introduced into the beam line in the calculation of the respective partial pressure values in the beam line.